GaP tensile strain compensation (SC) layers were introduced into GaAs solar cells enhanced with a five layer stack of InAs quantum dots (QDs). One sun air mass zero illuminated current-voltage curves show that SC results in improved conversion efficiency and reduced dark current. The strain compensated QD solar cell shows a slight increase in short circuit current compared to a baseline GaAs cell due to sub-GaAs bandgap absorption by the InAs QD. Quantum efficiency and electroluminescence were also measured and provide further insight to the improvements due to SC.

Abstract. We present a model addressing the possible electrification of Martian dust storms based on the effective electrical charging of an individual dust grain. An upper charge bound on a grain can be determined based on the grain capacitance in the lowpressure Martian atmosphere. It is assumed that treiboelectric and inductive processes, like that presumed operating in terrestrial dust storms, can electrify the grain to significant levels. A collection of such grains charged in a dust cloud of many tens of kilometers in size can yield a substantial electric field moment. Given various grain charge and dust storm sizes, the electric moment will be determined along with estimates of electrical discharge and emitted radio power based upon known models. We also suggest the possibility that remote detection of discharge-related VLF emission propagating in the surface/ionosphere waveguide can be used to determine subsurface conductivity. However, to date, there has been no report of orbiter or lander optical images of lightning-like discharges. Further, there is no report of lightning-induced interference on radio 3795

Single junction InGaP/GaAs solar cells displaying high efficiency and record high open‐circuit voltage values have been grown by metal–organic chemical vapor deposition on Ge/graded SiGe/Si substrates. Open‐circuit voltages of 980 mV under AM0 conditions have been verified to result from a single GaAs junction, with no evidence of Ge‐related sub‐cell photoresponse. AM0 efficiencies close to 16% have been measured for a large number of small‐area cells, the performance of which is limited by non‐fundamental current losses due to significant surface reflection resulting from> 10% front‐surface metal coverage and wafer handling during the growth sequence for these prototype cells. It is shown that at the material quality currently achieved for GaAs grown on Ge/SiGe/Si substrates, namely a 10 ns minority‐carrier lifetime that results from complete elimination of anti‐phase domains, and maintaining a threading dislocation density of ∼8 × 105 cm−2, 19–20% AM0 single‐junction GaAs cells are imminent. Experiments show that the high performance is not degraded for larger‐area cells, with identical open‐circuit voltages and higher short‐circuit current (due to reduced front metal coverage) values being demonstrated, indicating that large‐area scaling is possible in the near term. Comparison with a simple model indicates that the voltage output of these GaAs‐on‐Si cells follows the ideal behavior expected for lattice‐mismatched devices, demonstrating that unaccounted‐for defects and issues that have plagued other methods to epitaxially integrate III–V cells with Si are resolved by using SiGe buffers and proper GaAs nucleation methods. These early results already show the enormous and realistic potential of the virtual SiGe substrate approach for generating high‐efficiency, lightweight and strong III–V solar cells. Copyright © 2002 John Wiley & Sons, Ltd.

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The minority carrier lifetime of electrons (τn) in p-type GaAs double heterostructures grown on GaAs substrates and compositionally graded Ge/Si1−xGex/Si (SiGe) substrates with varying threading dislocation densities (TDDs) were measured at room temperature using time-resolved photoluminescence. The electron lifetimes for homoepitaxial GaAs and GaAs grown on SiGe (TDD∼1×106 cm−2) with a dopant concentration of 2×1017 cm−3 were ∼21 and ∼1.5 ns, respectively. The electron lifetime measured on SiGe was substantially lower than the previously measured minority carrier hole lifetime (τp) of ∼10 ns, for n-type GaAs grown on SiGe substrates with a similar residual TDD and dopant concentration. The reduced lifetime for electrons is a consequence of their higher mobility, which yields an increased sensitivity to the presence of dislocations in GaAs grown on metamorphic buffers. The disparity in dislocation sensitivity for electron and hole recombination has significant implications for metamorphic III-V devices.

Recent experimental measurements have shown that in GaAs with elevated threading dislocation densities (TDDs) the electron lifetime is much lower than the hole lifetime [C. L. Andre, J. J. Boeckl, D. M. Wilt, A. J. Pitera, M. L. Lee, E. A. Fitzgerald, B. M. Keyes, and S. A. Ringel, Appl. Phys. Lett. 84, 3884 (2004)]. This lower electron lifetime suggests an increase in depletion region recombination and thus in the reverse saturation current (J0 for an n+∕p diode compared with a p+∕n diode at a given TDD. To confirm this, GaAs diodes of both polarities were grown on compositionally graded Ge∕Si1−xGex∕Si (SiGe) substrates with a TDD of 1×106cm−2. It is shown that the ratio of measured J0 values is consistent with the inverse ratio of the expected lifetimes. Using a TDD-dependent lifetime in solar cell current–voltage models we found that the Voc, for a given short-circuit current, also exhibits a poorer TDD tolerance for GaAs n+∕p solar cells compared with GaAs p+∕n solar cells. Experimentally, the open-circuit voltage (Voc) for the n+∕p GaAs solar cell grown on a SiGe substrate with a TDD of ∼1×106cm−2 was ∼880mV which was significantly lower than the ∼980mV measured for a p+∕n GaAs solar cell grown on SiGe at the same TDD and was consistent with the solar cell modeling results reported in this paper. We conclude that p+∕n polarity GaAs junctions demonstrate superior dislocation tolerance than n+∕p configured GaAs junctions, which is important for optimization of lattice-mismatched III–V devices.

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Monolit hic Interconnected Modules (MIM) are under dev e lopment for thermophotovoltaic (TPV) energy conversion apphcations. MIM de vices are typifi ed by series-interconnected photovoltaic cells on a common , semi-insul ati ng su bstrate and generally include rear-surface infrared (lR) reflectors. The MIM arc hitecture is being implemented in InGaAsSb materials without semi-insulating substrates through the development of alternative isolation methodologies. Moti vations for developing the MIM structure include: reduced resistive losses , higher output power density than for systems utilizing fro nt surface spectral control, improved thermal coupling and ultimately higher system efficiency. Numerous design and material changes have been investigated since the introduction of the MIM concept in 1994. These developments as well as the current design strategies are addressed.